1. Field of the Invention
The present invention relates to a constant current circuit which is one of basic circuits in semiconductor integrated circuits, and particularly to a constant current circuit including a current feedback loop having a current mirror circuit of a band gap type.
2. Description of the Background Art
A semiconductor integrated circuit is provided with a constant current circuit for supplying each part of the circuit with a constant current. The constant current circuit of this type includes a starting circuit for ensuring operation of the circuit when a power supply is turned on.
FIGS. 3 and 4 are schematic diagrams of conventional constant current circuits each including a starting circuit.
First, the constant current circuit in FIG. 3 will be described. A PNP type transistor Q1, an NPN type transistor Q4 and a resistor R1 are connected in series between a power supply node 1 receiving a power supply potential and a ground node 2 receiving a ground potential. A PNP type transistor Q2 and an NPN type transistor Q5 are also connected in series between power supply node 1 and ground node 2. A PNP type transistor Q3, an NPN type feedback transistor Q6 and a resistor R3 are further connected in series between power supply node 1 and ground node 2. A PNP transistor Q9 is connected between power supply node 1 and an output node 3. A resistor R2 is connected in parallel to transistor Q6 between the collector of transistor Q3 and one end of resistor R3.
Transistors Q1, Q2 Q3 and Q9 have respective bases connected to each other, and connected to the collector of transistor Q3, the collector of transistor Q6 and one end of resistor R2. Transistor Q6 has its base connected to a node between the collectors of transistor Q2 and transistor Q5. Transistors Q4 and Q5 have respective bases connected to each other. Transistor Q4 has its base and collector short-circuited.
In the constant current circuit, transistors Q1, Q2, Q3 and Q9 constitute a first current mirror circuit .alpha.. Transistors Q4 and Q5 and resistor R1 constitute a second current mirror circuit .beta. of a band gap type. Resistors R2 and R3 constitute a starting circuit .gamma.2. Current mirror circuit .beta. of the band gap type is a circuit for conducting a current of a level determined based on a current density difference between two transistors Q4 and Q5 constituting current mirror circuit .beta. and a resistance value of resistor R1, to first current mirror circuit .alpha..
In the operation of the constant current circuit of FIG. 3, when the power supply is turned on, each base current of transistors Q1, Q2, Q3 and Q9 flows through resistors R2 and R3 to ground node 2, whereby a current flows from the collector of transistor Q2 to the base of feedback transistor Q6 to turn transistor Q6 on.
When feedback transistor Q6 is turned on, each base current of transistors Q1, Q2, Q3 and Q9 then flows through feedback transistors Q6 and resistor R3 to ground node 2, resulting in increase of the base currents and thus collector currents of transistors Q1, Q2, Q3 and Q9.
The increase in the collector current of transistor Q1 is followed by increase of a collector current of transistor Q4, resulting in increase of a base-emitter voltage of transistor Q4. Since the bases of transistors Q4 and Q5 constituting a current mirror circuit have equal potentials, a collector current of transistor Q5 increases with the increase in that of transistor Q4.
In second current mirror circuit .beta., when respective collector currents of transistors Q1 and Q2 increase to attain a stable current value determined based on a current density difference between transistors Q4 and Q5 of second current mirror circuit .beta. and a resistance value of resistor R1, the collector current of transistor Q2 flows into not only the base of feedback transistor Q6, but also transistor Q5.
Consequently, a base current and thus a collector current are made constant in feedback transistor Q6. When the collector current of feedback transistor Q6 becomes constant, the collector currents of transistors Q1, Q2, Q3 and Q9 increased in first current mirror circuit .alpha. become constant to make the constant current circuit stable as a whole, whereby a current provided from output transistor Q9 through output node 3 is made constant.
The constant current circuit in FIG. 4 will now be described. Since the basic structure of the constant current circuit in FIG. 4 is the same as that in FIG. 3, like numerals are attached to like parts, and the description thereof is not repeated. The difference between the constant current circuits in FIGS. 3 and 4 will be described in the following. The structure of a starting circuit .gamma.3 in the constant current circuit of FIG. 4 is different from that of starting circuit .gamma.2 in the constant current circuit of FIG. 3.
Starting circuit .gamma.3 comprises an NPN type transistor Q7 and a resistor R4. Transistor Q7 and resistor R4 are connected in series between a power supply node 1 and a ground node 2. Transistor Q7 has its base connected to respective bases of transistors Q1, Q2, Q3 and Q9 of first current mirror circuit .gamma..
In the operation of the constant current circuit of FIG. 4, when the power supply is turned on, each base current of transistor Q1, Q2, Q3 and Q9 flows through transistor Q7 and resistor R4 to ground node 2, whereby a current flows from the collector of transistor Q2 to the base of transistor Q6 to turn transistor Q6 on. Thereafter, the same operation as in the constant current circuit of FIG. 3 is carried out, so that the constant current circuit in FIG. 4 is made stable, ensuring the operation thereof.
In the constant current circuits shown in FIGS. 3 and 4, however, there exist the following problems.
In such a constant current circuit, after manufacturing the same, a test is made on the function of the circuit, where the constant current circuit is operated on trial for evaluation of the quality of the product based on the operation result.
In each of the constant current circuits in FIGS. 3 and 4 a wiring for an attached part tends to be disconnected, because it is a semiconductor integrated circuit having a structure in which the wiring is provided by vapor deposition of aluminum. For example, disconnection tends to occur at the points e and f in the constant current circuit of FIG. 3, and at the points g, h and i in the constant current circuit of FIG. 4. If such disconnection occurs in the constant current circuit of FIG. 3 or 4, feedback transistor Q6 is not usually turned on, and the circuit does not operate. Therefore, it is possible to determine the presence or absence of a defective part therein.
However, if the power supply potential suddenly rises on turning on the power supply, or if noise enters the circuit, a very small leak current flow through transistor Q2 might be generated. This could cause feedback transistor Q6 to be momentarily turned on, allowing the base currents to flow through transistor Q1, Q2, Q3 and Q9. As a result, the constant current circuit is started to operate, despite the disconnection in the circuit.
As described above, the conventional constant current circuit might operate despite disconnection therein, because of a leak current induced by noise. In order to exclude this possibility, several measures to eliminate noise are taken in a test apparatus for testing the circuit, such as provision of a shield preventing noise from entering the circuit, as well as gradual raise of a voltage immediately after turning on the power supply. However, all noise in the circuit can not be eliminated at the time of testing by the test apparatus since many parts are attached to the circuit to be tested.
Therefore, a problem exists that disconnection in a constant current circuit is difficult to be detected in the prior art, which leads to shipment of defective circuits. Accordingly, if such circuit having a disconnected portion is actually used by a user, disadvantage might occur that a circuit which operated in the test does not operate in the actual use, depending on the presence or absence of noise entering the circuit, and the conditions on which the circuit is used, such as the temperature of the environment. In addition, a problem exists that stricter measures against noise will lead to increase of time and costs required for the test, and reduction of a shipment rate.